1. Field of the Invention
The present invention relates to superconducting wires and, also, to a method and apparatus for fabricating such wires using a cylindrical hollow cathode magnetron sputtering system for depositing a film or films on cylindrical substrates.
2. Description of Prior Art
The phenomena of superconductivity, i.e., the ability to conduct electricity with almost no resistance, was first discovered in 1911 and has been the subject of intense scientific curiosity ever since. It has been known for many years that a number of elements, alloys and compounds, when cooled below a critical temperature approaching absolute zero (0 K), enter into a zero electrical resistance state. The superconducting transition temperature, or Tc, depends on the particular superconducting material or superconductor. When a superconductor is at a temperature which is higher than its critical temperature, it does not conduct electricity in an efficient manner in that some of the electrical energy is converted to heat. Such heat losses, however, may be reduced or eliminated when the superconductor is cooled below its critical temperature and the superconductor becomes a most efficient conductor of electricity.
In the past, only metallic superconductors having relatively low critical temperatures were generally available. Such superconductors include niobium-tin (NbSn) and niobium titanium (NbTi) superconductors The low transitional temperatures of these superconductors (near 20 K) dictated the use of liquid helium, which has a boiling point of about 4.2 K, as a coolant. Unfortunately, liquid helium cooling is very expensive, not only because helium is a costly, rare resource, but also because liquification of helium requires a large scale system. Thus, the use of superconductors of this type was very limited given the difficulty and expense of maintaining the temperature of superconductors below their critical temperatures.
Recently, however, ceramic oxide materials have been produced which exhibit superconductivity at much higher temperatures than the previous metallic superconductors. The critical temperatures of these newly-discovered ceramic superconductors are generally about 40 K, and in some instances even higher than 77 K, the boiling point of liquid nitrogen. Such ceramic superconductors can be cheaply maintained below their critical temperatures using inexpensive liquid nitrogen for cooling. The ability to produce superconductivity in a material cooled by liquid nitrogen completely changes the economics which have heretofore restricted the applications to which the phenomena of superconductivity could be applied.
The total number of applications for this new class of high temperature superconductors (HTS) can be stretched as far as the human imagination goes and may include the field of electrical power transmission, for example, MHD power generation, fusion power generation, power transmission and reservation, etc.; the field of transportation, for example, magnetically levitated vehicles, magnetically propelled chips, etc.; the medical field, for example, high energy beam radiation, etc.; the scientific field, for example, very sensitive sensors for detecting a very weak magnetic field, etc.; and the electronics field, for example, high speed/low power switching devices.
While the range of possible uses for the new HTS ceramic oxide materials is large and varied, a serious problem exists which could impede the full realization of those uses. Since the new HTS materials are oxides, they are inherently brittle and difficult to form directly into useful components such as wires, ribbons, tapes, fibers, or composites for the fabrication of superconducting devices.
Heretofore, the prevalent method used to produce the HTS ceramic oxides has been to mechanically mix powders of dioxides or carbonates of a rare earth, i.e., a lanthanum series element (such as lanthanum or yttrium); an alkaline earth metal element (such as barium or strontium); and copper in the 1-2-3 structure of the superconductor; calcine the mixture to remove water or other volatiles; and then fire the powder mixture in an oxygen atmosphere at a temperature sufficiently high to produce the desired superconducting phase. The shortcomings of this technique, however, are evidenced by variations in the compositions of the fired ceramic material and, consequently, variations in the chemical and physical properties of the resulting superconductors. Moreover, the mixing process requires several hours and sometimes introduces impurities from the mixing vessel (usually a ball mill) into the ceramic mixture.
As mentioned above, the HTS ceramic materials are deficient in some of the essential physical properties needed to permit ready fabrication and practical usage of structures made from such materials. Most notable of these deficiencies are the extreme brittleness and poor mechanical strength of the superconducting ceramic structures, which inhibit formation of shaped structures, such as coils or wires, and the low current carrying capabilities of the superconducting ceramic. The superconducting ceramic material also exhibits signs of microcracking which are a further indication of its brittleness and would also affect its critical current density Jc. In addition, the superconducting ceramic material, as generally produced, is of rather low density and is difficult to densify, resulting in low environmental stability and sensitivity to moisture and CO.sub.2. Low density also leads to poor superconducting and mechanical properties.
Several methods for the fabrication of HTS wires have been developed in an attempt to overcome the deficiencies of HTS ceramic materials described above. In one method, one or more precious metals selected from the class consisting of silver, gold, and one of the six platinum metals are added to the ceramic materials. Although this method produces a cermet having greater strength and flexibility than ceramic material, the maximum current densities are decreased. In the most popular method, the ceramic superconducting oxide powder is enclosed in a metallic (silver or stainless steel) sheet which is heat treated to a suitable high temperature. After heat treatment, the composite is cold or hot drawn or spun to reduce its diameter so as to provide an elongated superconductor wire with a desired diameter. Then the wire is subjected to a thermal process with a temperature higher than 900.degree. C. for a few hours. This latter method also has certain drawbacks. The wire made this way is not very flexible. Moreover, the difference between the thermal coefficient of expansion (TCE) of the superconductor and the metal sheet generates very high stresses at the interface during and after the high temperature heat treatment. This stress is usually tensile on the superconductor, thus generating microcracks and catastrophic failures in the oxide superconductor. The tensile stress in the oxide also results in a very low current density.
Other methods have formerly been developed in order to allow the practical use of superconducting materials. Several attempts have been made in the past to apply a thin coating of superconducting material to a base wire to fabricate a superconducting wire. One such technique, known as film deposition, involves supplying component HTS materials for a growing layer from external sources and depositing those materials down upon a substrate. Such deposition processes are generally carried out in a vapor phase within a reduced pressure atmosphere of a selected gas or gases, or in a vacuum. If the material to be deposited does not react chemically during deposition, the process is referred to as Physical Vapor Deposition or PVD. If, on the other hand, the deposited material is a product of a chemical reaction which occurs within the vapor phase, either on the surface or in the vicinity of the substrate, the process is known as Chemical Vapor Deposition or CVD. Hybrid methods of film deposition, i.e., those which involve both physical and chemical processes, are also known.
One method of physically depositing a film upon a substrate is known as sputtering. A typical sputtering system includes a target (a cathode) and a substrate holder (an anode) positioned so that the surface of a substrate upon which the film to be deposited, which substrate is placed on the holder, faces the target. The target is a plate of the material to be deposited or from which a film is to be synthesized. The target is connected to a negative voltage supply, either dc or rf, and the substrate holder may be either grounded, floating, or biased, as well as either heated, cooled, or some combination thereof. A gas, at a pressure from a few millitorr to about 500 mTorr, is introduced into a chamber containing the substrate holder and target to provide a medium in which a glow discharge plasma can be initiated and maintained. When the glow discharge is started positive ions strike the target and stimulate the removal of mainly neutral target atoms therefrom by momentum transfer. These atoms then condense into a thin film formed upon the surface of the substrate placed on the substrate holder. In addition, various particles other than neutral atoms, e.g., electrons and ions, are also produced at the target which may have a significant effect on the properties of the film deposited on the substrate.
Examining the sputtering process in more detail, a low pressure abnormal negative glow plasma discharge is maintained within the chamber between the cathode (target) and the anode (substrate holder). Electrons emitted from the cathode due to ion bombardment thereof are accelerated to near the full applied potential within the cathode dark space, i.e., a relatively nonluminous region between the cathode and the negative glow. Such high energy electrons enter the negative glow as so-called primary electrons where they collide with gas atoms and produce the ions required to sustain the plasma discharge. The primary electron mean free path increases with both increasing electron energy and decreasing pressure within the chamber. At low pressures, ions are produced far from the cathode where their chances of being lost are great. Additionally, many primary electrons hit the anode with high energies, causing a loss that is not offset by impact-induced secondary emission. Thus, ionization efficiencies are low. As the pressure within the sputtering chamber is increased at a fixed voltage, the primary electron mean free path decreases and larger currents are possible; however, at high pressures within the chamber the sputtered atom transport which occurs has been found to be reduced by collisional scattering.
It has also been found that a magnetic field extending parallel to the cathode surface can restrain primary electron motion to regions in the vicinity of the cathode and thereby increase ionization efficiency. It has been further found that the E.times.B electron drift currents can be caused to close on themselves by the use of cylindrical cathodes, which thereby prevent the E.times.B motion from causing the plasma discharge to be swept to one side. Based upon the foregoing, various cylindrical magnetron systems have been developed. Such systems having cylindrical, hollow cathodes are known as inverted magnetrons or cylindrical hollow magnetrons. A typical cylindrical hollow magnetron sputtering system includes one or more solenoids, wound on a core of magnetic material, and placed coaxially and externally to or within the cathode to serve as a field generator. Typically, the anodes are also joined to tubular backstrap and are both made from magnetic material. The aforementioned anode design effectively reduces field curvature near the ends of the anode and also increases the magnetic field strength in the plasma located inside the cathode. Where a plurality of solenoids are used, current ratios of those solenoids may be controlled to provide a variety of field shapes. To avoid changes caused by unequal heating of the solenoids, they are also typically connected in series with one another.
Heretofore, cylindrical hollow magnetron systems have been recognized as useful for coating substrates of complex shapes where: (a) the hollow cathode has a uniform wall erosion rate; (b) the substrate surface is far enough from the ends so that end losses can be ignored; and (c) the object to be coated has an unobstructed view of the cathode surface. Thus, heretofore, the usefulness of cylindrical hollow magnetron sputtering systems has been viewed as involving positioning the anode where end losses may be ignored and positioning the object to be coated so that it is always completely exposed to the cathode surface.
The deposition of thin film coatings onto cylindrical substrates such as wires and fibers, which have not been recognized as having complex shapes, has heretofore involved either rotation of the substrate while moving it relative to a uni-directional coating material source, such as a planar diode, or other steps wholly unrelated to the processes described herein. Needless to say, those prior processes which involve rotating a wire or fiber being coated with a thin film require the use of complex rotating means. Even using the most precise rotating systems now available cannot ensure a film of sufficient uniformity of thickness and quality for a number of currently developing applications. For example, it is becoming highly desirable to be able to deposit films of a few microns in thickness upon optical fibers, ceramic fibers, thin wires and other such cylindrical substrates. Certain new applications require the deposit of one or more films of metallic, superconducting, dielectric, electro-optic, magnetic and/or piezo-electric materials onto the surface of fibers and wires in highly precise and uniform layers.
In addition to the problems of film thickness and uniformity discussed above, the prior art methods of film deposition include a number of other shortcomings which render them inefficient in coating cylindrical substrates. For example, it has been found that using a planar magnetron sputtering system to apply relatively thick films over very large lengths of fibers is extremely inefficient because of the inherently low cathode material utilization characteristic of such systems when thin fibers are used as substrates.
The method and system of the present invention overcomes many of the disadvantages of prior art sputtering systems when the substrates to which a film is to be applied are wires and fibers.